1. Field of the Invention
The present invention relates to an exposure method and apparatus, and more particularly, to a scanning exposure method and apparatus for use in photolithography processes in the manufacture of semiconductor devices, image pickup devices (such as CCD), liquid crystal display devices, thin-film magnetic heads, and the like.
2. Discussion of the Related Art
In a photolithography process in the manufacture of semiconductor devices, an exposure apparatus has been used for projecting a circuit pattern on a mask (also referred to as xe2x80x9creticlexe2x80x9d) onto a substrate (also referred to as xe2x80x9cwaferxe2x80x9d or xe2x80x9cphotosensitive substratexe2x80x9d). In order to transfer a large circuit pattern with high accuracy and without increasing a burden on a projection optical system of the exposure apparatus, a so-called step-and-scan type exposure apparatus (or scanning exposure apparatus) has been recently developed. In the scanning exposure apparatus, the reticle and wafer are scanned in synchronization with each other with respect to an exposing radiation flux. A portion of the reticle pattern is thus illuminated so that the entire reticle pattern is successively projected onto each short area of the wafer through the projection optical system in a scanning manner. During the exposure operation of the scanning exposure apparatus, the reticle stage and the wafer stage need to be synchronously moved relative to each other at respective constant speeds.
In a conventional scanning exposure apparatus, after reticle alignment and wafer alignment are performed the reticle stage and wafer stage are moved to respective run-up start positions. Then, the reticle and wafer stages are accelerated in the respective scanning directions up to respective target speeds. After maintaining the target speeds for a predetermined settling time, which is required to settle synchronization errors between the reticle stage (reticle) and the wafer stage (wafer) within a predetermined tolerance, the reticle and wafer stages are irradiated with exposure light at the edge of the reticle pattern and the edge of the shot area on the wafer.
As described above, the reticle and wafer stages need to travel over certain run-up distances from the respective run-up start positions before starting actual exposure. In the conventional the step-and-scan exposure apparatus, the run-up distances are determined by adding a certain margin to the sum of an acceleration distance required for acceleration of each stage and a distance over which each stage travels during the worst (longest) settling time required and accordingly is fixed to a constant value. The run-up start positions of the stages are then determined such that the stages reach their respective exposure start positions after traveling the thus determined run-up distance. Accordingly, scanning exposure is started after ensuring the synchronization errors between the reticle and wafer are held within the fixed tolerance for each shot area.
However, in most cases, the actual time (settling time) necessary for the synchronization errors to settle within the fixed tolerance after the wafer and reticle stages reach their target speeds is not constant, but depends on various conditions, such as the position of a shot area (position of wafer stage), scanning speeds, and scanning directions of the reticle stage and wafer stage, etc. When the run-up distance is determined based on the worst value of the settling time and the run-up start position of each stage is uniformly determined based on the run-up distance, the run-up distance is unnecessarily greater than the actually required run-up distance for many shot areas. This results in a low throughput (low productivity).
In the step-and-scan type scanning exposure apparatus, in particular, since scanning exposure is carried out for each of many shot areas on the wafer, the exposure time tends to be longer than that of stationary-type exposure apparatus, such as a step-and-repeat exposure apparatus (so-called stepper). Accordingly, there is a need to further improve the throughput of the scanning exposure apparatus.
Accordingly, the present invention is directed to a scanning exposure method and apparatus that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a scanning exposure method that makes it possible to reduce run-up distances of stages for scanning exposure without increasing synchronization errors, thereby improving the throughput.
Another object of the present invention is to provide a scanning exposure apparatus that can carry out such a scanning exposure method.
A further object of the present invention is to provide a scanning exposure apparatus in which the run-up distance for each shot area is optimized to thereby improve the throughput while assuring high exposure accuracy.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a scanning exposure method according to a first aspect of the present invention includes the steps of accelerating a mask and a photosensitive substrate up to respective predetermined scanning speeds, starting projecting a part of a pattern formed on the mask onto the photosensitive substrate while a synchronization error between the mask and the photosensitive substrate is held in a predetermined tolerance, and moving the mask and the photosensitive substrate relative to each other so that the pattern of the mask is sequentially transferred to each shot area on the photosensitive substrate. In this scanning exposure method, when an exposure operation is performed with respect to a first shot area of a first photosensitive substrate, an acceleration settling time that is required for the synchronization error to settle within the tolerance is measured from a point of time when the mask and the photosensitive substrate start being accelerated, and stored, and when an exposure operation is performed with respect to a second shot area of a second photosensitive substrate, acceleration starting positions of the mask and the second photosensitive substrate are determined based on the acceleration settling time measured with respect to the first shot area.
According to the present invention, where the acceleration settling time measured for the first shot area is relatively short, the acceleration starting positions of the mask and the second photosensitive substrate are set to be relatively close to their exposure start positions. Namely, the run-up distances for the mask and photosensitive substrate may be shortened by learning the acceleration settling time from actual measurements, while ensuring that the synchronization error settles within the tolerance at the exposure start positions. This leads to reduction in the exposure time and improves the throughput. In this case, where the first photosensitive substrate is identical to the second photosensitive substrate, the acceleration settling time measured for the shot area(s) of one photosensitive substrate that has/have been exposed to light may be used for determining the acceleration starting positions (run-up distances) for the next shot area of the same substrate that is to be exposed to light.
When the first photosensitive substrate is different from the second photosensitive substrate, on the other hand, the acceleration starting positions (run-up distances) for each shot area of the second photosensitive substrate in a certain lot may be determined based on the acceleration settling time measured for all of the shot areas of the first photosensitive substrate in the same lot. In another method, the acceleration starting positions (run-up distances) for a shot area located at a certain position of the second photosensitive substrate may be determined based on the acceleration settling time measured for a shot area located at the same position of the first photosensitive substrate.
In the above case, the acceleration starting positions of the mask and the second photosensitive substrate may be determined based on the acceleration settling time, such that each of the acceleration starting positions is located ahead of a corresponding exposure start position with a spacing that is obtained by adding a spacing that provides a given margin to a distance over which each of the mask and the photosensitive substrate travels within the acceleration settling time. The distance that provides the given margin corresponds to a maximum of measured values of variations in the acceleration settling time at the same scanning speed as the first photosensitive substrate, for example. Thus, the synchronization error can surely be settled within the tolerance by the time the second shot area of the second photosensitive substrate reaches the exposure start position.
The first photosensitive substrate and the second photosensitive substrate may be the same photosensitive substrate. In this case, the run-up distance for the shot area that is to be exposed next can be shortened, as compared with that for the shot area of the same photosensitive substrate that has been exposed to light.
According to a second aspect of the present invention, the present invention also provides a scanning exposure apparatus including a mask stage for moving a mask and a substrate stage for moving a photosensitive substrate, wherein the mask and the photosensitive substrate are accelerated up to respective predetermined scanning speeds, a part of a pattern formed on the mask is projected onto the photosensitive substrate while a synchronization error between the mask and the photosensitive substrate is held in a predetermined tolerance, and the mask and the photosensitive substrate are synchronously moved relative to each other so that the pattern of the mask is sequentially transferred to each shot area on the photosensitive substrate. This exposure apparatus includes control means for measuring, when an exposure operation is performed with respect to a first shot area of a first photosensitive substrate, an acceleration settling time that is required for the synchronization error to settle within the tolerance from a point of time when the mask and the photosensitive substrate start being accelerated, and storing the acceleration settling time, and computing means for determining, when an exposure operation is performed with respect to a second shot area of a second photosensitive substrate, acceleration starting positions of the mask and the second photosensitive substrate, based on the acceleration settling time measured with respect to the first shot area. The scanning exposure method of the present invention is carried out by the scanning exposure apparatus constructed as described above.
In a third aspect, the present invention provides a scanning exposure apparatus including a mask stage that holds a mask and is movable in a scanning direction, a substrate stage that holds a substrate to be exposed to light such that a pattern on the mask is projected onto the substrate, the substrate stage being movable in the scanning direction and a non-scanning direction perpendicular to the scanning direction, and a stage control unit that accelerates the mask stage and the substrate stage from run-up start positions up to respective predetermined scanning speeds, settles a synchronization error between the mask stage and the substrate stage within a predetermined tolerance, and then controls constant-speed movements of the mask stage and the substrate stage that are kept in synchronization with each other. The scanning exposure apparatus further includes storage means for storing in advance a settling time required for settling the synchronization error to within the tolerance or coefficients that numerically represent influences on the settling time, for each of parameters that influence the settling time, wherein the stage control unit reads out the settling time or coefficients from the storage means depending upon exposure conditions, prior to exposure of each shot area on the substrate to be exposed, and change the run-up start positions according to the settling time or coefficients.
In the arrangement as described above, the storage means stores in advance the settling time required for the synchronization error between the mask stage and substrate stage to be settled to within the tolerance, or coefficients that numerically represent influences on the settling time, for each of parameters that influence the settling time or settling distance. In the actual exposure operation, the stage control unit reads out the settling time or the coefficients that numerically represent the influences on the settling time from the storage means depending upon exposure conditions, prior to exposure of each shot area on the substrate to be exposed, and changes the run-up start position of each stage according to the information thus read. Since the run-up start positions of both the mask and wafer stages are determined based on the settling time corresponding to the exposure conditions for each shot area, the run-up distance for each shot area can always be controlled to an optimum value. This leads to an improved throughput, as compared with the known example in which the run-up distance (or run-up period) is uniformly determined based on the worst or longest settling time. Also, the exposure accuracy does not deteriorate since the synchronization error has been held within the tolerance when the exposure is started.
A typical parameter in the above case that influences the setting time is the position of the stage, namely, the position of the shot area on the substrate to be exposed. In addition to the position of the shot area, parameters desirably include the scanning speeds of the mask stage and substrate stage and the scanning directions of the mask stage and substrate stage relative to the projection optical system.
Generally, the scanning speed needs to be changed depending upon an exposure amount that has been set, while the settling time varies depending upon the scanning speed. According to the present invention, the storage means stores in advance the settling times or coefficients that numerically represent influences on the settling time for different scanning speeds. Thus, even when exposure conditions, such as the exposure amount set, is changed and the scanning speed is accordingly changed, the stage control system reads out from the storage means the settling time or coefficients that numerically represent influences on the settling time depending upon the scanning speed thus changed. The stage control system changes the run-up start position according to the settling time or coefficients. Thus, the run-up distance is set to an optimum value.
The mask stage and substrate stage are often driven with different characteristics depending upon the scanning directions thereof, and the settling time varies depending upon the scanning directions. According to the present invention, the storage means stores in advance the settling time or coefficients that numerically represent influences on the settling time for different scanning directions. Accordingly, the stage control system reads from the storage means the settling time or coefficients that numerically represent influences on the settling time depending upon the scanning direction during exposure of each exposure area, and changes the run-up start position according to the information thus read, so as to establish an optimum run-up distance.
In a fourth aspect, the present invention provides a scanning exposure apparatus including a mask stage that holds a mask and is movable in a scanning direction, a substrate stage that holds a substrate to be exposed to light such that a pattern on the mask is projected onto the substrate, the substrate stage being movable in the scanning direction and a non-scanning direction perpendicular to the scanning direction, and a stage control unit that accelerates the mask stage and the substrate stage from run-up start positions to respective predetermined scanning speeds, settles a synchronization error between the mask stage and the substrate stage to within a predetermined tolerance, and then controls constant-speed movements of the mask stage and substrate stage so that they are synchronized. The scanning exposure apparatus further includes a host device that gives an exposure command to the stage control unit, prior to exposure of each shot area on the substrate to be exposed, the host device having information on a settling time that is measured for each shot area under predetermined exposure conditions to represent a period of time required for settling the synchronization error within the tolerance, the exposure command given to the stage control unit including the information on the settling time for each shot area.
In the scanning exposure apparatus as described above, the host device has information indicating the settling time required for the synchronization error to be settled to within the tolerance measured for each shot area under predetermined exposure conditions. Before exposure of each shot area in the actual exposure operation, the hose device generates an exposure command including the settling time for the relevant shot area to the stage control unit, and the stage control unit changes the run-up start positions of both stages based on the information indicating the settling time included in the exposure command. Thus, the run-up start positions of both of the stages are determined based on the information indicating the settling time included in the exposure command for each shot area, and therefore the run-up distance for each shot area can be always controlled to an optimum value. This leads to an improved throughput, compared to the conventional apparatus in which the run-up distance (or run-up period) is always uniformly determined based on the worst or longest settling time. Further, the exposure accuracy does not deteriorate since the synchronization error has been held within the tolerance by the time exposure is started.
The scanning exposure apparatus according to this aspect of the present invention is particularly suitable when exposure conditions for all of the shot areas are predetermined, and exposure operations are repeatedly performed for a given period of time without changing the exposure conditions of all of the shot areas.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.